Second-order nonlinear optical (NLO) chromophores and polymers are being actively pursued for applications in high-speed electro-optic (EO) modulators, and other integrated optoelectronic and microwave devices [(a) Kajzar, F.; Lee, K.-S.; Jen, A. K.-Y. Adv. Polym. Sci. 2003, 161, 1. (b) Dalton, L. Adv. Polym. Sci. 2002, 158, 1. ] Since the EO effect is not a naturally occurring property in polymers, the poling under an electrical field is needed to induce the linear EO coefficient, which is parallel to an applied field as given by:χzzz(2)=Nβ111fω2f02μE/5kT 
where N is the density of NLO chromophore, β111 is the first hyperpolarizability of the molecular chromophore in the direction of μ, fω2 and f02 are local field correction factors at frequencies ω and zero respectively, μ is the ground state dipole moment of the chromophore, E is the applied electric field and T is the temperature. Simple translation of these molecular values into bulk property values assumes that all the molecules represented in the number density (N), respond to the local poling field (E) at a temperature (T). The electro-optic coefficient (r33) can be given as:r33.≈2/n4Nf02β<cos3 θ>
To be practically used in high-performance or new EO devices, NLO polymeric materials should have large and stable EO response or r33 values, for example, much larger than that (r33=31 pm/V) of LiNbO3 crystals currently used in commercial EO modulators, at the wavelengths of 1310 nm, 1485-1525 nm, 1525-1562 nm and 1565-1620 nm, as prescribed by the International Telecommunications Union. Furthermore, it is desirable to maximize N and the product of dipole moment (μ) and molecular hyperpolarizability (β) or μβ. The former factor (N) is determined by the number of active NLO chromophores in the polymer and the latter (μβ) is a property of the chromophore. Accordingly, ideal chromophores for EO applications should have large hyperpolarizability (β) such as those having extended π-conjugated systems or large dipole moment (μ) or both, and should be readily soluble in solvents and polymer matrices. However, extended conjugation in molecules tends to lead to poor thermal and photochemical stability. The large dipole moment is likely to cause strong dipole-dipole interaction between chromophores, which is a result of stacking the two chromophores in a head-to-tail fashion. Thus, such a dipole interaction can lead to the diminished dipole orientation or even completely cancellation of dipole orientation of chromophores during the poling process, which gives rise to low or no EO response.
Equally importantly, suitable NLO materials must be able to be formulated and standardized to fulfil the processing requirement for device development and fabrication. For most developmental work on EO devices [(a) Shi, Y.; Zhang, C.; Zhang, H.; Bechtel, J. H.; Dalton, L. R.; Robinson, B. H.; Steier, W. H. Science 2000, 288, 119. (b) Lee, M.; Katz, H. E.; Erben, C.; Gill, D. M.; Gopalan, P.; Heber, J. D.; McGee, D. J. Science 2002, 298, 1401. (c) Paloczi, G. T.; Huang, Y.; Yariv, A.; Luo, J.; Jen, A. K.-Y. Appl. Phys. Lett. 2004, 85, 1662. ], the guest-host type of NLO materials are routinely employed. However, phase separation over time and poor temporal stability of the poled NLO materials are common as reported in literature, due to poor solubility and facile dipole relaxation of chromophores in a non-crosslinked polymer matrix. Linking the chromophores via covalent bonds onto linear or hyperbranched macromolecules, which include oligomers and polymers in any molecular weights, can effectively increase the chromophore loading, prevent phase separation and stabilize the dipole orientation [(a) Burland, D. M.; Miller, R. D.; Walsh, C. A. Chem. Rev. 1994, 94, 31. (b) Bai, Y.; Song, N.; Gao, J. P.; Sun, X.; Wang, X.; Yu, G.; Wang, Z. Y. J. Am. Chem. Soc. 2005, 127, 2060. ]. However, chromophores must be properly functionalized for incorporation into linear and hyperbranched macromolecules by grafting or polymerization.
A large number of chromophores have been synthesized and some exhibit very large macroscopic nonlinearities in guest/host polymers and grafted polymers. A strong ‘push-pull’ molecule with both electron accepting and electron donating groups linked by a conjugated moiety usually shows a finite P value. For example, chromophore-1 reported by Dalton et al (Opt. Lett, 1998, 23, 478) is a neutral molecule with relatively small dipole moment and contains several double bonds; DEMI reported by Szablewski et al (J. Am. Chem. Soc. 1997, 119, 3144) is a charged molecule or zwitterionic and has just one bridging double bond and fairly large dipole moment (e.g., 45 D). Zwitterionic chromophores are promising as NLO molecules, because their strongly asymmetric conjugated structures result in both a large hyperpolarizability and dipole moment. They are also deemed to be more stable than the neutral chromophores, due to less labile double bonds. The aligned or poled zwitterionic chromophores in a polymer are calculated to have extremely high EO coefficients (r33>210 pm/V, Cross, G. et al. Opt. Mater. 2002, 21, 29. ). To unlock the potential of the class of zwitterionic chromophores for EO device applications, one must find a way to effectively reduce the dipole-dipole interaction.

Some zwitterionic chromophores structurally similar to DEMI but containing the pyridinium moiety and the hydroxy groups have been disclosed (Wang, Z. Y., et al. U.S. Pat. No. 6,894,169). These PQDM chromophores have the formulae:

DEMI and PQDM have large dipole moments and large β values. But they also show poor solubility and strong dipole-dipole interaction. As a result, only a very small amount (1-3 wt %) of each chromophore could be doped into a host polymer without severe phase separation. The hydroxy-containing PQDM can be introduced into a host polymer via a covalent bond, but when PQDM chromophores in the polymer are more than 10 wt %, the EO coefficients drop due to increased dipole-dipole interaction. PQDM chromophores lack of proper substituents at the pyridinium and phenylene units to allow for reduction of the dipole-dipole interaction. None of three cyano groups could be further chemically modified or converted into other functional groups, such as an ester group, by any known chemical transformations without chemically damaging the other parts of the PQDM molecules. The disclosed PQDM chromophores have not reached and will not reach the high EO coefficients as expected for this class of zwitterionic NLO chromophores.